The comparison

Every common mechanism, on the properties that actually differ.
Type What it senses Accuracy Drift Response Used for
Capacitive Polymer film capacitance ±2–5% RH ~1%/year Seconds Nearly everything
Resistive Salt or polymer resistance ±5% or worse Faster Tens of seconds Low-cost devices, HVAC switching
Hair tension Hair length ±5–10% Substantial Minutes Antique and decorative dials
Psychrometer Evaporative cooling ±2–3% None inherent 1–2 minutes Field reference, calibration
Chilled mirror Dew forming on a cooled surface ±0.2°C dew point Minimal if kept clean Seconds to minutes Laboratory reference standard
Gravimetric Mass of water extracted Primary standard Not applicable Hours National metrology institutes

Reading the table

Accuracy is not the whole story. A capacitive sensor at ±3% that has been on a shelf for four years may be further out than its specification suggests, while a psychrometer at ±3% will still be ±3% in a decade because nothing about it drifts — it is a thermodynamic relationship rather than a calibrated material.

That distinction is the main reason psychrometers survived so long in professional use, and why they still appear in calibration procedures.

Response time matters more than people expect. A hygrometer that takes ten minutes to settle is useless for walking around a building looking for a damp spot, however accurate it eventually is.

Drift is what actually ruins consumer readings. Roughly a percent a year for a decent capacitive sensor, faster for resistive, much faster for hair. After five years, an uncalibrated device may be well outside anything it claimed on the box — which is why the salt test is worth twenty minutes of your time.

What each is actually for

  • Capacitive — everything, unless you have a specific reason otherwise. Cheap, fast, stable enough, no maintenance beyond calibration.
  • Resistive — switching something on and off, where a few percent does not matter.
  • Hair tension — an object on a wall. Charming, historically important, and not a measurement instrument in any modern sense.
  • Psychrometer — checking whether your other instruments are lying, and any situation where a traceable reading matters more than convenience.
  • Chilled mirror — laboratory and industrial work where the dew point itself must be known precisely. Expensive, and the mirror needs cleaning.
  • Gravimetric — how the entire chain of calibration is ultimately anchored. Not something you will operate.

The one that is not an instrument

There is a seventh source of humidity readings and it belongs in any honest list: a weather service.

It is not a mechanism but a network — real instruments from the rows above, distributed across a region, combined with model output and interpolated to a point. That is what a forecast, this website's live pages and the app all report.

Its error profile is completely different from a handheld instrument's. Sensor accuracy is excellent, because official stations are well maintained; but the nearest one may be miles away, and distance is the dominant error. It is the right tool for outdoor questions and no tool at all for indoor ones.